At One Place

Stopping distance at every speed

Stopping distance is two things added together
How far you travel before you touch the brake, plus how far you travel after
The first is proportional to speed. The second is proportional to speed squared — which is why doubling your speed more than doubles the distance.

Published stopping-distance tables disagree with each other, and the reason is always the same: they quietly choose different reaction times and different tyre friction figures. This one uses 1.35 seconds, which is the middle of the measured range for an unanticipated hazard, and a friction coefficient of 0.7 for dry asphalt. Both are stated so you can see what changes when they change.

The arithmetic

  • Reaction distance = speed × reaction time. At 100 km/h that is 27.8 m/s × 1.35 s = 38 metres before the brake is touched.
  • Braking distance = v² ÷ (2 × μ × g), the standard constant-deceleration result, assuming the tyres stay at the friction limit — an ABS car in good condition on a level road.
  • μ is the tyre-road friction coefficient: about 0.7 on dry asphalt, 0.4 wet, 0.2 on packed snow, 0.1 on ice. g is 9.81 m/s².
  • Only the braking term is squared. That is why the total rises far faster than the speed does.

On dry asphalt

Reaction time 1.35 s, μ 0.7, level road, ABS, good tyres. Worn tyres or a downhill grade lengthen the braking term substantially.
Speed Reaction distance Braking distance Total
20 km/h — 12 mph 8 m 2 m 10 m — 32 ft
30 km/h — 19 mph 11 m 5 m 16 m — 54 ft
40 km/h — 25 mph 15 m 9 m 24 m — 79 ft
50 km/h — 31 mph 19 m 14 m 33 m — 108 ft
60 km/h — 37 mph 23 m 20 m 43 m — 140 ft
70 km/h — 44 mph 26 m 28 m 54 m — 176 ft
80 km/h — 50 mph 30 m 36 m 66 m — 216 ft
90 km/h — 56 mph 34 m 46 m 79 m — 260 ft
100 km/h — 62 mph 38 m 56 m 94 m — 307 ft
110 km/h — 68 mph 41 m 68 m 109 m — 358 ft
120 km/h — 75 mph 45 m 81 m 126 m — 413 ft
130 km/h — 81 mph 49 m 95 m 144 m — 471 ft

What the surface does

Total stopping distance, same reaction time throughout. Ice roughly triples the dry-road figure.
Speed Dry asphalt Wet asphalt Packed snow Ice
20 km/h 10 m 11 m 15 m 23 m
30 km/h 16 m 20 m 29 m 47 m
40 km/h 24 m 31 m 46 m 78 m
50 km/h 33 m 43 m 68 m 117 m
60 km/h 43 m 58 m 93 m 164 m
80 km/h 66 m 93 m 156 m 282 m
100 km/h 94 m 136 m 234 m 431 m
120 km/h 126 m 187 m 328 m 611 m

Why the two-second rule works

Following distance measured in seconds rather than car lengths automatically scales the reaction component with speed, which is exactly the part that scales linearly. It does nothing for the braking component, which is why the advice becomes four seconds in the wet and considerably more on ice — you are buying margin for the squared term.

These figures assume you brake at the friction limit immediately. Real drivers frequently do not brake hard enough in the first second of an emergency, which is what brake assist systems exist to correct.

Common questions

What is the stopping distance at 100 km/h?
About 94 metres on dry asphalt — 38 m of reaction distance and 56 m of braking. On a wet road it is about 136 m.
Why does doubling speed more than double stopping distance?
Because only the reaction component scales with speed. The braking component scales with speed squared, since kinetic energy does. Doubling from 50 to 100 km/h roughly quadruples the braking distance.
Why do published stopping distance tables disagree?
They choose different reaction times — anywhere from 0.7 to 2.5 seconds — and different tyre friction coefficients. Both assumptions move the answer a long way, and most tables do not state either.

Related pages

Sources

  1. Typical range compiled from manufacturer specifications — At One Place
  2. A Policy on Geometric Design of Highways and Streets (the Green Book) — American Association of State Highway and Transportation Officials
  3. Calculated on this page — At One Place

How these figures are compiled and checked